Keep local release binary out of fast builds

[cbusillo]

Summary

• stop dev-fast builds from rewriting code-rs/target/release/code
• make just local-code-rebuild replace a stale release-bin symlink with a real release binary
• add an explicit helper to remove the old manual /opt/homebrew/bin/code symlink
• document the local binary ownership flow

Validation

• just local-code-rebuild
• ./build-fast.sh
• code --version reports code 0.6.102 after both commands
• code update-check --tag v0.6.102 reports status: up to date
• removed local stale /opt/homebrew/bin/code symlink via just local-remove-homebrew-code-link